An image forming device by an electrophotographic method, such as a copier, printer, facsimile, or multi-function center having various functions of these devices, forms a latent image on a photoreceptor, corresponding to an original document sheet, develops the latent image by adding toner, transfers the developed toner image onto a recording medium, fixes the toner image transferred on the recording medium, and ejects the recording medium.
To form a color image, an image forming device forms four latent images in Y (yellow), M (magenta), C (cyan), and K (black) colors, corresponding to colors on an original document sheet, on a photoreceptor drum, primarily transfers developed toner images of four colors onto an intermediate transferrer constructed with an endless belt, then secondarily transfers a composite toner image of the four toner images onto a recording medium, fixes the toner image transferred onto the recording medium, and ejects the recording medium.
Further, as a fixing device for fixing toner images in such a way, there is a fixing device of a belt fixing type which includes a fixing belt that is supported with tension by a heating roller with a built-in heater, such as a halogen heater, and a fixing roller, and is driven in circulation. The fixing device further includes a pressure roller for pressing the fixing roller through the fixing belt. The fixing device heats and presses a recording medium, onto which a toner image has been transferred, at a nip section formed by the fixing belt and pressure roller, while sandwiching and conveying the recording medium. Such a fixing device, including a fixing belt and heating roller with small thermal capacities, has advantages of shortening warming-up time and energy saving.
However, a conveying device employing an intermediate transferrer, fixing belt, and recording medium conveying belt in an endless form has a problem that an endless belt supported with tension by rollers is displaced to one side or the other of the rollers or obliqued. If a side face of a belt is received by a fixed shaft or a belt guide member of a rigid body that does not move along the roller axis, a problem tends to occur in that the pushing force applied to the side face of the belt becomes large, and the belt gets on the belt guide member or the side face of the belt gets broken.
As a solution of the problems described above, an example in a prior art is shown in FIG. 6 where displacement of a fixing belt is restrained. FIG. 6 shows an example of a fixing device in a prior art having a structure to restrain displacement of an endless fixing belt. As shown in FIG. 6, disclosed is a technology for absorbing displacement and obliquity of a belt 927 such that a spring 912 receives a belt-guide-lib 911 (belt restrain member) disposed on a side face of a roller 910 near the end thereof. (For example, refer to Japanese Patent Publication TOKKAI No. 2003-241554)
However, the following problem has not been solved even by this method. That is, for example, when a jam of a recording medium has occurred on the fixing belt 927, a large pushing force toward a side face of the roller 910 is generated on the fixing belt 927. Consequently, the side face of the fixing belt 927 gets on one of the belt restraining members 911 provided at the both ends of the roller 910 and gets deformed, or a side end portion of the fixing belt 927 enters the gap between the roller 910 and the belt restraining member 911, thus the fixing belt 927 being damaged.
In an aspect of the present invention, there is provided the following structure.
An image forming apparatus, comprising:
a plurality of rotors; and
an endless belt that is supported with tension by the rotors and that is rotated by the rotors,
wherein at least one of the plurality of rotors is provided at a first end with a belt restraining member that has a restraining face for restraining displacement of the endless belt toward the first end and that is freely rotatable with respect to the rotor; and
wherein the belt restraining member is arranged such that an angle between the restraining face of the belt restraining member and a rotation shaft of the rotor is greater at a contact start point at which the endless belt begins contact with the belt restraining member during rotation than at a contact end point at which the endless belt ends contact with the belt restraining member during rotation.